"Be on side" means to be supportive or in agreement with someone or a particular cause. The antonyms for this phrase include "be against," "oppose," and "disagree." When someone is not "on side" with you, they do not share the same viewpoint or goals as you do. They may actively work against you or simply not lend their support to your cause. It is important to note that being on the same side as someone does not necessarily mean agreeing with them on every aspect or decision. It simply means working together towards a shared goal or objective.